1. Field of the Invention
The present invention relates to a production control system suitable for use in manufacturing plants wherein a great variety of parts and assemblies are assembled to form products, such as automobiles.
The present invention also relates to a production schedule planning system for a mixed production line and particularly when a mix of different types of products are produced by assembling parts delivered from respective operation tables arranged along an assembly line, the delivery sequences of the parts are restrained and equated in a predetermined manner.
2. Description of the Prior Art
An automobile is an assembly of a great number of parts. In general, the production process for automobiles comprises the steps of assembling individual parts to form a sub-assembly (for example, an engine) and then incorporating that sub-assembly into another assembly (for example, a vehicle body). Such steps are repeated with other parts and sub-assemblies to complete an automobile as a final product. The recent development of producing various automobile models complicates the process by requiring a wide variety of parts having variable specifications for successive sub-assemblies at a given operation table.
In the accompanying drawings, FIG. 1 shows a model of a production process wherein many types of cars are to be produced. For example, in considering which parts or assemblies are to be incorporated into vehicle bodies AM, it is found that a plurality of engines EG/1-EG/N may be incorporated into the automobiles or vehicle bodies AM and that each of the engines EG/1-EG/N are themselves composed of plural parts a-n.
The assembly operations of these parts a-n and engines EG/1-EG/N usually are not performed on a single production line to obtain final products; instead the assembly operations are mostly separated into specialized production lines. This specialization necessarily involves transportation of parts or engines between a production factory and temporary storage spaces making it difficult to have a smooth flow of parts or engines on the production line. It is well-known in the art that some production control is desired.
FIG. 2 illustrates a set of specialized production lines for producing various types of automobile sub-assemblies. An engine producing factory 30 produces engines EG/1-EG/N as outgoing products on a production line 31 with the use of individual parts a-n which have been supplied from separate parts factories. The various types of engines EG/1-EG/N will not move on the production line 31 in a uniform manner, but engines of different types will be fed at a rate corresponding to the number of engines of each type required by one of the other assembly factories (hereinafter called the "required number"). Therefore, the required number corresponds to the rate of production of each type of engine on the production line 31 per unit time. Each of the engines EG/1-EG/N emerging from the production line 31 is subjected to an inspection step 32 and then stored temporarily in a storage space 33.
Thereafter, the required number N(i, j) of engines EG/1-EG/N which are classified into the different engine types j/1-j/n, are transported to each of the destined factories (vehicle factories) i/1-i/n by any suitable transporting means such as trucks or the like.
Each of the destined factories i/1-i/n will then produce the desired automobiles by incorporating the engines EG/1-EG/N into the vehicle bodies that are on its own production line (41 or 42) in order to match the production schedule of that factory.
In such a production system, the sub-assemblies must be supplied to the destined factories in the proper quantities so that the required types of automobiles will be smoothly produced in each factory by the required delivery time. To this end, automobile manufacturing factories utilize production controls based on data relating to the delivery of parts,i the number of parts or sub-assemblies required by each of the destined factories, as well as other data.
The general production control system 50 is a concentrated control system using a computer and a processing program which has previously been prepared by inputting necessary data into the computer through a console 51. FIG. 22 shows a flow chart of a conventional production control program. A production control system in the prior art will now be described with respect to the production of engines EG/1-EG/N.
In FIG. 22, data relating to the required number of engines that are to be produced in an engine producing factory 30 (hereinafter called "production factory sometimes) is inputted in the computer through the console (Step 100). Various productive conditions (including the type of engine, the number of cars to be produced and other factors) relating to the engine producing factory 30 are then inputted into the computer (Step 101). Based on this inputted data, the required number of engines are edited and classified into the required types of different products (Step 102). Subsequently, the productive conditions are processed one at a time (Step 103) and the production order of the different sub-assemblies that are to be produced 31 (that is, the productive order) is passed along to the leading end of the production line (Step 104). The engines are produced on the production line 31 in accordance with such instructions. Thus, a plurality of engine types will move on the production line 31 at a predetermined rate of emergence.
The aforementioned production control system of the prior art has the following problems. First, the number of engines that must be produced,i which is inputted at Step 100, is determined by the arrival time of the engines at each of the destined factories (i/1-i/n). As shown in FIG. 2, the destined factories are all at different locations. Thus,i different periods of time are required to transport the engines from the engine producing factor 30 to the destined factories i/1 -i/n. In spite of this added complexity, the prior art did not take it into account when determining the required number of engines for each destination. As a result, when the number of engines to be produced is calculated, according to the schedule of the particular engine factory, a discrepancy will be created with respect to time. Thus, the overall number of engines required by all the destined factories i/1-i/n will not necessarily coincide with the overall number of engines produced ion the engine producing factory 30. This results in the under-production or the over-storage of the engines EG/1-EG/N in the storage space 33 of the engine producing factory 30. This discrepancy will also influence the delivery of parts 1-n, creating as unstable production condition such that the parts will be supplied to the engine factory 30 in an excessive or insufficient amount.
The second problem in the prior art production control system is that the progress of production in each of the production factories, including the engine factory 30, is not taken into consideration. If any undesirable event (such as a malfunction in the production facilities) takes place in the engine factory 30 or one of the other destined factories i/1-i/n, the required number of engines inputted at step 100 cannot be updated, and production will continue without a rapid increase of the total stock in the storage space 33 of the engine producing factory 30. Whatever the case may be, the engine producing factory 30 may over-produce only one specific type of engine and thus make the production and physical distribution unstable.
The third problem in the prior art is that a plurality of productive conditions are only processed one at a time at Step 104. Engines EG/1-EG/N should be produced on the production line 31 at a rate matched to the schedule of production. For example, if the proportion of the produced engines EG/1 to EG/2 is 1 : 1, the engines EG/1 and EG/2 should emerge alternately on the production line 31. However, some event may occur so that two engines of type EG/2 are successively produced after only one engine EG/1 has emerged from the production line. This result is a consequence of the fact that cumulative errors will occur if many different types of engines are to be produced on the same production line 31, because the ratio of the different types of engines produced will not be an integer number. Subsequent sequence of production is determined from the previous sequence and thus the next type of engine to be produced is determined by comparing the rates of production with each other.
In order to apply the recent mass-production system used for producing various types of products, a mixed production line is used in practice on which products of different types or different specifications are assembled, in contrast to the prior ar job-lot production.
Such a mixed production line may be used for the manufacture of various products, including automobiles, engines, electric home appliances and the like.
In particular, automobiles or engines thereof, require a great number of different parts which are to be assembled and which should be properly supplied in accordance with a predetermined production schedule. In such cases, it is very important that the assembling sequence of the different parts be appropriately established.
If the delivery of parts is not properly performed, either a shortage of parts will be created, or over-storage of parts will occur, or there will be other inconveniences. These undesirable events result in a decrease in the production efficiency of the production line or the parts manufacturing factory.
Because the mixed production line has an increased variability with respect to the time required to assemble or work different parts, it is preferable that those parts requiring a longer assembly or work time and those requiring a shorter assembly or work time be supplied to the production line in an alternate manner as much as possible, or, alternatively, at a constant rate.
Such a condition in the assembling operation is known as a restraint condition. For example, if electronic fuel injectors (EFI) and carburetors are manufactured on the same engine assembly line, a restraint condition inhibiting the successive assembly of EFI's is required since the assembling of EFI's requires a very long period of time. An assembly line that successively produces only some EFI's is undesirable.
On the other hand, the parts manufacturing section is preferably required to produce various types of parts in as equal a number as possible. It is also desirable to provide an assembly line on which the various types of parts can be combined and arranged so that they will be assembled with equal frequency.
Such a requirement from the parts manufacturing section is known as an equating condition. It is desirable that the permutations and combinations of parts supplied to the respective operation tables satisfy the equating condition.
FIGS. 3 exemplifies a mixed production line in which a rotary 52 forms an assembly line along which a number of operating tables A, B, C . . . are located. Each of the tables receives parts from an associated source. When the assembly line completes a rotation, a product will be assembled which is determined by the parts supplied. If the parts are supplied to the respective operating tables in a preselected order, it is possible to mass-produce products with the desired specifications in a mixed assembly line operation.
It is usually preferred that a group of parts which are classified into the same category (for example, in the automobile assembly line), normal carburetor and EFI parts be supplied to the same operating table.
The aforementioned rotary 52 normally includes a given number of operation tables around it. For example, one rotary includes 100 tables. Such a rotary can repeatedly produce 100 products which have different specifications by repeating a delivery cycle to the rotary that corresponds to one assembly line cycle, for example, a delivery cycle through which 100 parts are supplied to the assembly line.
As previously described in connection with FIG. 1, by considering which parts or assemblies are incorporated into an automobile AMI, it is realized that one of a plurality of engines EG/1-EG/N may be incorporated into each of the automobiles AM/l and that each of the engines EG/1-EG/N is composed of a plurality of parts a-n. In the production process, various types of parts are combined with one another, therefore, the delivery of parts should be properly influenced by the various models of automobiles having different specifications that are to be produced on a single assembly line.
As is well-known in the art, the assembly of parts a-n and engines EG/1-EG/N are not performed on a single production line. The assembly operations are usually separated into specialized production lines. The specialization of production lines is necessarily accompanied by a transportation of parts or engines between each manufacturing plant and a temporary storage space. Such a factor prevents the smooth flow of parts and engines to the production lines. It is well-known in the art that some production control is desired.
Normally, parts are supplied to a production line from a plurality of manufacturing factories or storage spaces. The parts should be delivered in the proper quantities in a manner matching their movement along the assembly line. At this time, the requirement of parts on the assembly line may not necessarily coincide with their delivery rate on the production side. This is a very important problem affecting the sequence of parts supplied to the operating tables around the assembly line that also affects both the producing and assembling sections.
In view of both the production and storage sides, it is preferable to equate the required frequency of emergence of the parts on the mixed production line, thereby improving the efficiencies of the storage and production facilities. This will be apparent by considering an example where a production schedule is used in which only specific parts are consumed for one day during a week, parts producing facilities must be enlarged and storage spaces must be correspondingly increased.
The equating requirements of the parts in such a mixed production line are extremely important in order to increase the overall efficiency of production.
On the other hand, it is preferable that the time required to assemble or work parts on the operation tables be equal since the assembly line or rotary 52 moves at a constant speed, as is apparent from FIG. 3. However, the mixed production line cannot usually have such equal times. As described before, the equality cannot be prevented also by a difference between times required to produce the carburetors and times required to produce EFI's.
As described before, it is necessary that parts having different operational difficulties be supplied in such a way as to equalize the overall operational time in the production line. In practice, such a condition can be fulfilled by supplying parts requiring a difficult operation in a sequence that is not successive, that is, by fulfilling the aforementioned restraint condition.
As is well-known, the production of automobiles is executed on a production schedule developed for a specific day. This daily schedule is planned by considering the monthly, weekly and daily production schedules in conjunction with data obtained up until that day, data such as production results and the like. A parts delivery program will be planned based on this planned production schedule. For example, if a production schedule for 1500 automobiles per day is planned with respect to a specific assembly line, and assuming that this assembly line includes 100 operating tables, the desired production is accomplished by fully rotating the assembly line 15 times per day. Parts are supplied to the assembly line in a manner reflecting the different specifications allocated to the 1500 automobiles to be produced.
In FIG. 3, the final products are shown as a combination of "a/1, b/1, c/1 . . . . " or "a/2, b/2, c/2 . . . . " each of which are used to specify a particular part.
Accordingly, the combination of parts or units (hereinafter called "combination code") required is first established based on the production schedule. The arrangement and row order are then established with respect to these combination codes in order to determine a sequence or production order for products having different specifications. Subsequently, by specifying the parts that are to be delivered to their respective tables in accordance with this established sequence of combination codes, the desired product can be assembled during the movement of the rotary 52 through a complete rotation.
In the schedule planning method of the prior art, however, both the aforementioned equating and restraint conditions could not be fulfilled since all the necessary factors were incorporated into a given period of production, such as a daily production schedule, for example. The arrangement and sequence of the combination codes used to determine the final production plan was determined only by using one of the above conditions and this was typically the equating condition.
The equating condition requires that the necessary parts emerge from the production line with equal frequency as much as possible. On the other hand, the restraint condition relates to the periods of time required at each of the operating tables during the assembling or working of the parts. It is known in the art that the equating and restraint conditions are frequently inconsistent with each other.
Since no combination or row order which can fulfill both these inconsistent conditions can be obtained, the prior art selects and uses only one of the two conditions and usually the equating condition is chosen.
When only the equating condition is used to perform a mixed production in which products consisting of many common parts are to be produced, the sequence of production will be upset if the daily production schedule is highly varied.
Because of the preferential use of the equating condition. The restraint condition is not usually fulfilled. This means that work of greater complexity is successively effected near the end of the production process.
Furthermore, the equating condition is set for a constant period (for example, a daily production cycle) that is independent of the assembly line cycle. As a result, the operating tables will operate according to different parts delivery cycles A row order of parts equal to, for example, 1500 in number must be set for each of the operating tables. This requires a huge arithmetic or data storing operation.